Surface reformation method of high polymer material

ABSTRACT

There is disclosed a surface reformation method of a high polymer material such that by irradiating and excimer-laser beam to only a predetermined area in which electronic parts and the like are temporarily immobilized by a liquid on a substrate which has a high polymer layer on the surface, wettability of the liquid for temporary immobilization only with respect to the predetermined area is improved. After the electronic parts are temporarily immobilized on the substrate by using the method, the electronic parts can be soldered with preferable durability by a fluxess reflow soldering.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a surface reformation method ofa high polymer material and, more particularly, to a surface reformationmethod of a high polymer material which is preferable to be used inorder to improve wettability of a liquid used for temporarilyimmobilizing electronic parts on an electronic circuit board of whichsurface is covered by a high polymer material layer. The invention alsorelates to improvement of wettability of a liquid used for temporaryimmobilization of electronic parts when the electronic parts aresoldered on an electronic circuit board according to a fluxless reflowsoldering.

[0002] As a conventional technique regarding a surface reformationmethod for wettability improvement of a high polymer material, forexample, methods using an O₂ asher, Ar sputter, or the like are known.In the methods, a process is performed by setting a circuit board onwhich a high polymer material layer to be processed is coated in avacuum vessel, and the surface reformation is executed for the entiresurface of the high polymer material on the substrate.

[0003] According to the conventional technique using, for example, theO₂ asher or Ar sputtering mentioned above, the surface of the substratecan be reformed only in an atmosphere of vacuum and there is a problemsuch that a large-scaled apparatus including vacuum equipment, vacuumvessel, and the like is necessary. According to the conventionaltechnique, since the surface reformation is executed for the entiresurface of the high polymer material on the substrate, an area which isnot necessary to be processed other than the area on which theelectronic parts are temporarily immobilized on the electronic circuitboard or the like is also reformed. Consequently, the liquid used forthe temporary immobilization of the electronic parts wets the portionwhich is unnecessary for the temporary immobilization of the parts, sothat there is a problem such that a large amount of vain liquid isneeded. As mentioned above, when the entire surface of the high polymermaterial on the substrate is reformed and the liquid for temporaryimmobilization wets even the unnecessary portion, even if the parts aretemporarily immobilized, the parts are moved from positions of thetemporary immobilization at the time of a reflow soldering, so that theobject of the temporary immobilization cannot not be achieved.Particularly, when the size of the substrate is equal to 50 mm×50 mm orlarger, since the liquid for temporary immobilization tends to gather inthe center and the temporarily immobilized parts easily move, it is notpreferable. Hitherto, a flux for soldering is commonly used in thetemporary immobilization of the parts in the reflow soldering and theparts are not moved from the positions of the temporary immobilization.In case of performing the reflow soldering without using the flux,however, when the liquid for temporary immobilization wets theunnecessary portion, the object of the temporary immobilization is notachieved as mentioned above. It is, therefore, conventionally difficultto solder the electronic parts at predetermined positions on theelectronic circuit board by performing a fluxless reflow soldering.

SUMMARY OF THE INVENTION

[0004] It is an object of the present invention to provide a surfacereformation method of a high polymer material which solves the problemsof the conventional technique and which can perform a surfacereformation of a substrate in which wettability for a liquid employedfor temporary immobilization is improved with respect to only an areanecessary for temporary immobilization of electronic parts withoutneeding vacuum equipment, vacuum vessel, and the like. The surfacereformation here denotes the reformation of the properties of thesurface for improving the wettability of the liquid for temporaryimmobilization.

[0005] The above-mentioned object of the present invention is achievedby irradiating only the area to be treated of the surface (that is, anarea to be wetted by the liquid for temporary immobilization) of highpolymer material as a target with light energy.

[0006] According to the present invention, by applying the light energyto only the area which needs the surface reformation of the present highpolymer material, the surface of the only portion which is irradiatedwith the light energy can be reformed and the wettability for the liquidcan be improved only in the necessary area. Consequently, the liquid fortemporarily immobilizing the electronic parts and the like can make onlythe processed area wet, the processed area can be wetted by the liquidof the minimum amount, and the electronic parts can be certainlytemporarily immobilized at predetermined positions on the surface of thehigh polymer material.

[0007] As a base material of a substrate on which the high polymermaterial layer is provided, any substrate used for an integrated circuitsuch as, for example, printed board, glass plate, or ceramic plate canbe used.

[0008] A polyimide system resin is used for the high polymer materiallayer on which the electronic parts are mounted. As the polyimide systemresin, polyimide isoindroquinazolindion (PIQ; trade name of HitachiChemical Co., Ltd.) and polymethylmethacrylate (PMMA) can be usuallyemployed. In addition, OFPR (trade name of Tokyo Ohka Kogyo Co., Ltd.),B20 (trade name of Hitachi Chemical Co., Ltd.), and the like can be alsogiven.

[0009] A wavelength of the light energy which is thrown onto the surfaceof the high polymer material lies in a range from 100 nm to 600 nm. Whenthe wavelength of the irradiation light is too short, the irradiationlight is transmitted to the inside of the high polymer material. Whenthe wavelength is too long, abrasion of the high polymer materialsurface is insufficient. Both of the cases are not preferable.

[0010] An energy density of the irradiation light is set to larger than0.03 J/cm² and is equal to 0.5 j/cm² or less. When the energy density isequal to 0.03 J/cm² or less, a contact angle of the liquid for temporaryimmobilization is equal to 20 degrees or larger, so that the improvementof the wettability is insufficient. When the energy density exceeds 0.5J/cm², it is not preferable since a damage caused to the resin layer istoo large to permit when the thickness of the polyimide system resinlayer is equal to 0.5 μm or less.

[0011] As light energy which satisfies the above conditions, anexcimer-laser beam can be used in the present invention.

[0012] Irradiation by the excimer-laser beam is performed by throwing apulse of 20 ns to 100 ns (pulse duration is 20 ns to 100 ns) at leastonce. The pulse duration is set to 20 ns to 100 ns, because the pulseduration of the excimer-laser beam is equal to from 20 ns to 100 ns atthe present technique level, it is not an absolute condition. The energyof each pulse of the irradiation pulse of the light energy exceeds 0.03J/cm² and is equal to 0.5 J/cm² or less. At least one pulse of the lightirradiation is necessary. Although the number of pulses can be one ormore, the cost increases when the number is set to an unnecessarilylarge number, so that it is usually set to 5 times or less.

[0013] As a liquid for temporary immobilization, for example, an alcoholsystem liquid such as tetraethylene glycol or pentaethylene glycol or anester system liquid such as ethyl salicylate can be used.

[0014] A proper amount of the liquid for temporary immobilization isadhered to a predetermined area by, for example, dropping the liquid inthe predetermined area. An adhesion amount of the liquid for temporaryimmobilization is set to an amount sufficient to wet the predeterminedarea on the surface of the high polymer material to temporarilyimmobilize the electronic parts and ordinarily set to 4.5 cc to 13.5 ccfor an area of 16 mm×16 mm (that is, 0.017 cc/mm² to 0.053 cc/mm²).

[0015] According to the method of the present invention, by temporarilyimmobilizing the electronic parts on the high polymer material layer onthe substrate, the electronic parts can be easily soldered at positionswhere the electronic parts are temporarily immobilized in a step of thefluxless reflow soldering after that.

[0016] In order to solder the electronic parts on the substrate, a metalpattern for the soldering can be provided in at least an area to besoldered on the substrate. The metal pattern in this case is constructedby a solderable metal (for example, Cu) and the thickness can be anythickness as long as it is easy to be soldered. The soldering can beperformed in accordance with, for example, the micro Ball Grid Array(micro BGA) method or CCB (Controlled Collapse Bonding) method. A metallayer of a predetermined shape is provided on the surface of the basematerial of the substrate and the high polymer material layer havingholes corresponding to predetermined positions of the metal layer iscoated on the substrate on which the metal layer is provided. On theother hand, a solder ball is provided for the electronic part at aposition corresponding to the hole section and the solder ball and themetal layer exposed at the hole section of the high polymer layer arealigned, thereby temporarily immobilizing the electronic parts.

[0017] In the method of the present invention using the irradiationlight of the energy density in the above range, the metal pattern forsoldering is not damaged and the surface reformation of the high polymermaterial as an insulating material can be performed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a schematic perspective view for explaining a surfacereformation method of a high polymer material in one embodiment of thepresent invention;

[0019]FIG. 2 is a section view showing a construction of a substrate inone embodiment of the present invention;

[0020]FIG. 3 is a section view showing a state in which electronic partsare temporarily immobilized on a substrate in one embodiment of thepresent invention;

[0021]FIG. 4 is a schematic perspective view showing a state in which ametal pattern is damaged because of the improper intensity of anirradiation light;

[0022]FIGS. 5a, 5 b, and 5 c are perspective views showing substrates towhich light energy is applied so that patterns of irradiation areas area square, a circle, a shape obtained by gathering a plurality ofpatterns on the substrate respectively;

[0023]FIG. 6a is an explanatory diagram showing a wet state by a liquidin one embodiment of the present invention;

[0024]FIG. 6b is an explanatory diagram showing a wet state by a liquidin the conventional technique;

[0025]FIG. 7a is a schematic perspective view showing a step of droppinga liquid for temporary immobilization onto a substrate in order toevaluate the wettability for the liquid used in one embodiment of thepresent invention;

[0026]FIG. 7b is a schematic perspective view showing a state in whichthe liquid is spread after the step of FIG. 7a;

[0027]FIGS. 7c, 7 d, and 7 e are schematic perspective views showingstates such that the liquid is further spread after the step of FIG. 7band the wet states of the liquid become to be evaluated;

[0028]FIG. 8a is a schematic section view for explaining a contact anglefor judging the wettability for the liquid in another embodiment of thepresent invention; and

[0029]FIG. 8b is a graph showing the relation between the contact angleof the liquid for temporary immobilization and the energy density of theirradiation light in the another embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] EMBODIMENT 1

[0031] Embodiment 1 will be described hereinbelow with reference to thedrawings. In the drawings, reference numeral 1 denotes a laserirradiation area; 2 a substrate; 3 a laser beam; 4 a high polymermaterial layer; 5 a base material; 6 an electronic part; 7 a liquid fortemporary immobilization; 8 a metal wiring; 9 an area worked by an O₂asher; 10 a pipette; 11 a wetted area; 12 a ball solder; 13 anelectrode; 14 a metal pattern; and 15 a hole section.

[0032] In the present embodiment, as shown by a construction example inFIG. 2, the substrate 2 to be processed in accordance with the surfacereformation method for the high polymer material is constructed in amanner such that the high polymer material layer 4 such as PIQ(polyimide isoindroquinazolindion), PMMA (polymethylmethacrylate), orthe like is coated as an insulating material on the surface of the basematerial 5 such as ceramic or the like. On such a substrate 2, metalwiring necessary for forming an electronic circuit is performed andelectronic parts such as LSI and the like are soldered at predeterminedpositions. The substrate 2 in the present embodiment has a square shapeof 150 mm×150 mm and the thickness of 5 mm. As shown in FIG. 3, the highpolymer material layer 4 is formed on the surface of the substrate basematerial 5 on which the metal wiring 8 is arranged. There are the holesections 15 at predetermined positions of the high polymer materiallayer 4. At the positions, the metal wiring 8 is exposed to form themetal pattern 14 to be soldered.

[0033] The present embodiment intends to improve the wettability of onlynecessary portion on the substrate 2 for the liquid 7 for temporaryimmobilization in case of temporarily immobilizing the electronic parts6 and the like by using the liquid 7 for temporary immobilization whenthe electronic parts 6 such as LSI and the like are soldered.

[0034] As shown in FIG. 1, the surface reformation method of the highpolymer material 4 according to the present embodiment is executed byapplying the light energy by the laser beam 3 to the laser irradiationarea 1 to which the electronic parts 6 are temporarily immobilized onthe substrate 2 which is coated by the high polymer material layer 4. Asa light source of the laser beam 3, an excimer-laser is used. It isparticularly preferable that the wavelength of the laser beam 3 lieswithin a range from 100 nm to 600 nm and the energy density lies withina range from 0.05 J/cm² to 0.5 J/cm².

[0035] When an alcohol system solvent such as tetraethylene glycol,pentaethylene glycol, or the like as a liquid 7 for temporarilyimmobilizing the electronic parts is dropped onto the laser irradiationarea 1 on the surface of the substrate 2 to which the surfacereformation has been performed by irradiating the laser beam 3, theliquid 7 evenly wets in the laser irradiation area 1 and does not wetthe unprocessed surface of the substrate 2. The electronic parts 6 suchas LSI and the like can be temporarily immobilized by the liquid 7 atthe predetermined positions on the substrate 2 to which an embodiment ofthe present invention has been performed before soldering as shown inFIG. 3.

[0036] Even in the case where the surface reformation method of the highpolymer material according to the present embodiment is performed to thesurface of the substrate 2 on which the metal pattern 14 exists for thesoldering, the surface of the high polymer material layer 4 coated onthe substrate 2 can be reformed without damaging the metal wiring 8.When the energy density of the laser beam 3 is remarkably larger thanthe values indicated in the invention, for example, equal to 20 J/cm²,the metal wiring 8 is damaged as shown in FIG. 4 and it can be broken.

[0037] Further, in the method according to the embodiment of the presentinvention, the shape of the laser irradiation area 1 can be optional inaccordance with the sectional shape of the laser beam 3 to be irradiatedas shown in FIGS. 5a, 5 b and 5 c. For example, it can be a square shownin FIG. 5a, a circle shown in FIG. 5b, and a shape obtained by gatheringa plurality of shapes for reforming a wide region shown in FIG. 5c. Themethod according to the present embodiment is effective in either one ofthe atmosphere, vacuum, He assist and can in such circumstance reformthe surface of the high polymer material layer 4. The He assist denotesan atmosphere during spraying He in order to blow off dusts. By sprayingHe, dusts can be blew off further than spraying other gas.

[0038] A state of wettability with respect to the liquid when the methodaccording to the foregoing embodiment is performed will be described incomparison with the case of the conventional technique with reference toFIGS. 6a and 6 b.

[0039] As shown in FIG. 6a, the liquid 7 as an alcohol system solvent incase of performing the method according to the embodiment wets only aportion of the area 1 which was irradiated by the laser beam. This isbecause the portion which is not subjected to the process according tothe present embodiment does not allow the liquid 7 to wet due to thesurface tension even when the liquid is dropped to the not processedportion or the liquid tries to invade from the area 1 to which the laserbeam has been thrown.

[0040] On the other hand, in case of the method according to theconventional technique using the O₂ asher, as shown in FIG. 6b, sincethe area 9 to be worked by the O₂ asher cannot be limited to a specificarea and the entire surface of the substrate is processed. The liquid 7consequently wets the whole surface of the substrate 2.

[0041] As mentioned above, in the present embodiment in which only thepredetermined area on the substrate 2 is set to the laser beamirradiation area 1, the liquid 7 for temporary immobilization wets onlythe laser beam irradiation area 1 as shown in FIG. 6a. The electronicparts 6 having the solder ball 12 on the electrode 13 as shown in FIG. 3are mounted on the substrate 2 of which only predetermined area is wetby the liquid 7 for temporary immobilization and the electronic parts 6are temporarily immobilized in a manner such that the solder ball 12faces the metal pattern 14 through the hole section 15 of the highpolymer material layer 4. The substrate 2 on which the electronic parts6 are set as mentioned above is reflow soldered, thereby soldering theelectronic parts to the predetermined positions on the substrate. Theflux is not particularly used. The residual liquid for temporaryimmobilization after the completion of the soldering is evaporated byputting the substrate in the vacuum atmosphere.

[0042] On the other hand, when the surface of the high polymer materialis not especially processed (when the irradiation of the light energy,the work by the O₂ asher, sputtering, and the like are not performed),the surface of the high polymer material is not wet by the alcoholsystem solvent, and the electronic parts cannot be temporarilyimmobilized, so that the electronic parts cannot be soldered by thefluxless reflow soldering.

[0043] When the surface of the high polymer material is processed by theO₂ asher or Ar sputtering as in the conventional technique, the liquidfor temporary immobilization spreads over the entire surface of the highpolymer material and the electronic parts cannot be soldered to thepredetermined positions on the substrate by the fluxless reflowsoldering as mentioned above.

[0044] EMBODIMENT 2

[0045] A method of evaluating the method according to the presentembodiment of the present invention will now be described with referenceto FIGS. 7a to 7 e.

[0046] First, the substrate 2 having the square of 150 mm×150 mm andthickness of 5 mm is prepared as a sample as shown in FIG. 2, and by theexcimer-laser beam having the wavelength of 308 nm, the energy densityof 0.1 J/cm², and a time of 30 ns is irradiated once at the area 1 ofthe square of 16 mm×16 mm. After that, as shown in FIG. 7a, the liquid 7of the alcohol system solvent, that is, tetraethylene glycol orpentaethylene glycol of about 4.5 cc to 13.5 cc, typically about 9 cc isdropped by using the pipette 10. The droplet wets the whole surface in apredetermined area as shown in FIG. 7b. The droplet spread by wetting isleft for 15 minutes. When the liquid 7 remains spread on the entiresurface of the predetermined processed area and is not leaked out of theprocessed area as shown in FIG. 7c, it is evaluated that the surface issufficiently reformed.

[0047] On the other hand, a case where it is evaluated that the surfaceis not sufficiently reformed is shown in FIGS. 7d and 7 e. FIG. 7d showsa state where the liquid 7 wets even out of the predetermined area 1.FIG. 7e shows a state where an area wetted by the liquid 7 is reducedand a portion which is not wetted by the liquid 7 appears on thepredetermined area 1. The surface reformation is insufficient in both ofthe cases.

[0048] In case of irradiating the surface of the high polymer materialby the light energy under the conditions of the present invention, apreferred result shown in FIG. 7c can be obtained.

[0049] EMBODIMENT 3

[0050] An experiment result for obtaining the optimum energy density ofthe excimer-laser beam according to the present embodiment will now bedescribed with reference to FIG. 8.

[0051] The surface reformation can be evaluated by measuring the contactangle θ as an angle between the surface of the liquid 7 for temporaryimmobilization and the surface of the substrate when the temporaryimmobilizing liquid is dropped onto the surface of the substrate 2 asshown in FIG. 8a. When the contact angle θ is equal to 20 degrees orless, it can be usually evaluated that the surface reformation regardingthe wettability is sufficient. As it is known, the contact angle ismeasured by photographing the side and measuring the contact angle inthe photograph. The surface of the substrate 2 is composed of PIQ. Theirradiation condition of the laser beam, kind of the temporaryimmobilizing liquid, and dropping condition of the temporaryimmobilizing liquid are the same as those in the embodiment 2. Theenergy density of the laser beam is changed in a range shown by the axisof abscissa of FIG. 8b.

[0052]FIG. 8b is the experiment result showing the relation between theenergy density of the excimer-laser beam and the contact angle θmentioned above.

[0053] As will be clearly understood from FIG. 8b, when the energydensity of the excimer-laser exceeds 0.03 J/cm², the contact angle θbecomes 20 degrees or less and the effect of the invention can beobtained. When the energy density is equal to 0.05 J/cm² or larger, thecontact angle θ is sufficiently smaller than 20 degrees and it isfurther preferable. When the energy density exceeds 0.5 J/cm², however,it is not preferable since the damage caused to the high polymermaterial layer becomes large as mentioned above, it is preferable thatthe energy density is equal to 0.5 J/cm² or less. When the energydensity exceeds 0.15 J/cm², there is a tendency such that a residueafter working increases, so that it is further preferable that theenergy density is set to 0.15 J/cm² or less.

[0054] In the present invention, the energy density of the excimer-laserbeam is consequently set to larger than 0.03 J/cm² and is equal to 0.5J/cm² or less and, preferably, it is equal to 0.05 J/cm² or larger andis equal to 0.15 J/cm² or less.

[0055] In the present embodiment, the pulse of the excimer-laser beamfor 30 ns is irradiated only once onto the substrate, thereby obtainingthe data. When the pulse is irradiated three times or when theirradiation time is set to from 20 ns to 100 ns, it is confirmed that analmost similar result can be obtained. Further, when PMMA is used for ahigh polymer material, a substantially same result can be also obtained.

[0056] When the laser irradiation condition by which the preferredresult is obtained in the present embodiment is applied to Embodiment 2,the preferred result can be also obtained.

[0057] According to the surface reformation method of the high polymermaterial according to the present invention as mentioned above, byapplying the light energy to only the area which needs the surfacereformation of the high polymer material, the surface reformation ofonly the portion to which the light energy is irradiated can beperformed and the wettability of the liquid can be improved only in thenecessary area. Thus, the liquid for temporarily immobilizing theelectronic parts and the like without needing the vacuum equipment orthe like can be wetted only in the process area and the process area canbe wetted with the minimum amount.

[0058] According to the invention, since the surface reformation of onlythe necessary area can be performed, the electronic parts and the likecan be temporarily immobilized on the substrate by using the minimumamount of the liquid for temporary immobilization.

[0059] When the surface reformation is executed by using the surfacereformation method of the high polymer material of the presentinvention, the electronic parts and the like are temporarily immobilizedon the substrate by using the liquid for temporary immobilization afterthat and the fluxless reflow soldering is performed, thereby enablingthe soldering of the electronic parts and the like on the substrate tobe executed with preferable durability.

[0060] In the surface reformation method of the high polymer material ofthe invention, even when the metal pattern for soldering exists in thelight energy irradiation area, the metal pattern is not damaged.

What is claimed is:
 1. A surface reformation method of a high polymermaterial for improving wettability of a liquid on the surface of a highpolymer material, wherein light energy is irradiated onto the surface ofthe high polymer material in a desired area of which said materialsurface is to be reformed.
 2. A method according to claim 1, whereinsaid high polymer material is polyimide system resin.
 3. A methodaccording to claim 2, wherein said polyimide system resin is polyimideisoindroquinaqolindion or polymethylmethacrylate.
 4. A method accordingto any one of claims 1, 2, and 3, wherein said light energy has awavelength which lies in a range from 100 nm to 600 nm and energydensity which exceeds 0.03 J/cm² and is equal to 0.5 J/cm² or less.
 5. Amethod according to claim 4, wherein said light energy is anexcimer-laser beam.
 6. A method according to claim 4, wherein the energydensity of said light energy is equal to 0.05 J/cm² or larger.
 7. Amethod according to claim 4, wherein the energy density of said lightenergy is equal to 0.15 J/cm² or less.
 8. A method according to claim 5,wherein said light energy is irradiated by irradiating a light energypulse, of which duration lies in a range from 20 ns to 100 ns, at leastonce.
 9. A method according to claim 4, wherein a metal pattern forsoldering is provided in said desired area of said high polymermaterial.
 10. A method according to claim 1, wherein said light energyis irradiated in either one of the atmosphere, vacuum, and He assist.11. A method according to claim 1, wherein said liquid is an alcoholsystem liquid or an ester system liquid.
 12. A method according to claim1, wherein said liquid is one liquid selected from the group consistingof tetraethylene glycol, pentaethylene glycol, and ethyl salicylate.